The present invention relates to a process of producing steel ingots particularly composite steel ingots having, at the skin, a steel composition which is different from that of the heart, a process in accordance with which an incompletely killed steel, mainly effervescing, rimming or semi-killed steel, is first of all treated in the tapping ladle, and it is then tapped into an ingot mould, if need be dead-headed, after filling the ingot mould and the passage of a predetermined time, a deoxidizing agent is added to the steel in the ingot mould and the said steel is subjected to mixing under the effect of a gas injected into the steel near the bottom of the ingot mould. The invention also includes such a steel ingot.
A known process of the type mentioned above has the disadvantage of not preventing, in the solidified ingot, the numerous non-metallic inclusions existing normally in the steel or the inclusions made up by the residues of deoxidation which are prejudicial to the use of the ingots obtained or which lead more or less to an important percentage of scrap.
It has already been proposed to effect the mixing of steel for an extremely short period of time, less than 30 secs, in order not to assist too much a rapid crystallization of the steel outside the solidification front, which crystallization, as is known, impedes or prevents the decanting of the inclusions at the surface of the ingot. Now it has been ascertained that a mixing of a very short duration such as proposed up to the present does not permit the efficient decanting of the deoxidation residues such as the oxides of aluminium and/or of silicon which are then of a volume which is too small to be able to decant rapidly.
In another process of the same type, with a view to ensuring a good homogeneity of the steel, it has been proposed to maintain the mixing of the steel by neutral gas until the said steel has become very rich in crystals. This manner of proceeding certainly reduces the number of inclusions, but those which remain entrapped are voluminous such that the ingots obtained are of mediocre quality.